R33_06980

The objective of this study is to understand the effect of Phosphorous (P) on precipitation and segregation behavior in neutron irradiated Reactor Pressure Vessel steels in the Advanced Test Reactor (ATR-2) using Atom Probe Tomography. Under normal condition, the RPV has sufficient fracture toughness however, in the irradiated condition, the fracture toughness of the RPV may be severely degraded due to precipitation of solute elements. The matrix features (MFs) formed from the cascades are known to produce hardening in RPV steels containing both low and high Cu content. The excess concentration of vacancies under irradiation also accelerates precipitation of Cu, along with Ni, Mn, P and Si which results in formation of copper rich precipitates (CRPs) and manganese-nickel rich precipitates (MNPs). At low P levels and typical low to intermediate flux irradiation conditions, the dominant MFs are believed to be Mn-Ni-Si-P-Cu vacancy solute complexes or their remnants. However, at higher P levels alloy phosphide precipitates are observed. The effects of P and role of phosphide phases are relatively poorly understood for irradiated RPV steels. Previous studies have shown that role of P in mediating hardening and embrittlement is greatly enhanced at higher Mn, due to formation of Mn3P precipitates. Although, the amount of impurity P in RPV alloys is generally very small, (< 0.05 wt %), it is very insoluble, and remains supersaturated following typical stress-relief heat treatments. Thus, P can undergo accelerated precipitation due to radiation enhanced diffusion (RED) to form phosphide phases. In order to investigate role of P in precipitation and segregation behavior, a series of RPV steels with varying solute contents were irradiated to a fluence of 1.4x1020 n/cm2 at Tirr of 290°C with flux of 3.6x1012 n/cm2 in the UCSB ATR-2 experiment at a typical LWR operating temperature of 290°. This proposal aims to use atom probe tomography (APT) to quantify the P enriched precipitates that formed under irradiation in this series of steels with wide ranges of Cu and Ni content